This invention relates to applying masking lines to porous capacitor elements without mechanical contact and with minimal material waste.
Solid capacitors contain elements fabricated from a porous valve metal foil and a conductive polymer counter electrode. The elements are stacked together to form the capacitor. FIG. 1 depicts several elements (1) attached to a process bar (6). Each element has an anode (2) and a cathode (3). Two masking layers (4) and (5) are also applied to the elements. The elements can be fabricated from any valve metal foil, typically tantalum, niobium, or aluminum. Aluminum is the preferred material.
FIG. 2 depicts a flowchart of the steps to manufacture the element and the final capacitor device. The foil is first etched (10) and formed (11) using conventional techniques such as by passing rolls of foil through etching and forming baths. These rolls are typically from a few hundred millimeters to a few meters wide. The formed foil is then slit (12) into narrower rolls the width of the final element. After this slitting, the foil is cut to a length convenient for further processing (13). The resulting elements are attached to a process bar for carrying them through the subsequent steps of processing (14). The material of the process bar is typically aluminum or stainless steel.
During the slitting and cutting to length operations, the dielectric oxide on the element edges is damaged and bare metal is exposed. If a cathode material is applied over the edges of the elements, a short circuit would occur and the capacitor would have an extremely high leakage current. Therefore, new dielectric oxide needs to be formed on the cut edges of the element. This can be accomplished by immersing the elements in a formation bath or a series of formation baths. However, if the elements attached to the process bars are simply immersed in the formation electrolyte, the electrolyte will wick up to the process bar. This wicking is undesirable because the material of the process bar will form an oxide instead of the edges of the foil and impurities from the process bar can contaminate the formation electrolyte. Thus a barrier needs to be placed between the top of the element and the process bar. This barrier can take the form of a non-conductive masking material (15).
After forming the edges of the element (16), the cathode material is applied (18). Typically, the cathode material is manganese dioxide or an intrinsically conductive polymer material. The cathode is conventionally applied by a series of dipping and heating operations. In order to prevent the cathode material from touching the anode portion of the capacitor and causing a short circuit, the anode portion of the capacitor needs to be separated from the cathode portion by a barrier layer. This layer can be the masking layer applied to prevent the electrolyte from wicking up to the process bar or more preferably can be an additional masking layer applied below the first masking layer (17).
After the application of the cathode material, carbon paint (19) and silver paint (20) are applied to complete the fabrication of the elements. The elements are then cut from the process bar (21). The cathode end of the element is attached to one side of a lead frame while the anode side of the element is attached to the other side of the lead frame (22). Optionally, multiple elements can be stacked on top of another to form a multi-layer capacitor. Finally, the elements are encapsulated in a non-conductive molding compound (23).
The need to provide an insulating barrier or masking layer was recognized by Harakawa, et al. (U.S. Pat. No. 4,805,074). Harakawa, et al. teach an insulating xe2x80x9cresistxe2x80x9d layer to separate the anode from the cathode, but do not provide a method of forming this layer.
More recently, Kuranuki, et al. (EP 0 634 761 and EP 0 634 762) proposed an adhesive insulating tape, such as a heat resistant polyimide tape, be applied as the insulating layer. A disadvantage of the adhesive tape is that the adhesive on the tape may not penetrate the pores of the foil and, therefore, allow material to wick up.
Monden, et al. recognize that application of a liquid resin followed by curing of this resin would provide penetration of the insulating resin into the pores and prevent the material from wicking up (Japanese Patent Application H11-123598). Nitoh, et al. propose to apply the resin using a metal wheel (Japanese Patent Application H11-123599). This method and apparatus require that the metal wheel be in contact with the capacitor element. This could cause damage to the dielectric oxide film and also potentially bend the element. Additionally, there is a restriction on how close the masking line can be to the process bar causing waste of metal foil. Furthermore, it is difficult to supply material to this wheel to provide only a small waste of masking material.
A method of preparing a capacitor having at least one porous element comprising applying to the element a masking material with an ink jet printer head. Preferably the masking material is a liquid resin such as an acrylic, a polyurethane, a silicone, or a polyimide.